Test Router serialization round-trip in functional_tests.

Test Router serialization round-trip in functional_tests.

This tests Router serialization round-trip at the end of each
functional test in the same way we do ChannelMonitors and
ChannelManagers to catch any cases where we were able to get into
a state which would have prevented reading a Router back off disk.

We further walk all of the announcements which both the original
and deserialized Routers would send to peers requesting initial
sync to ensure they match.